In the manufacture of chemical and food handling machinery incorporating component parts which function in movable metal to metal contact, it is often necessary to have material combinations that are at once corrosion resistant to the environment and exhibit non-galling and wear resistant properties. Of particular importance are parts such as pump impellers, pistons, sleeves, and bearings where sliding metal contact occurs, poor lubrication conditions exist, and parts are in intimate contact with a corrosive media, i.e., an acidic product being pumped, blended, conveyed or otherwise handled, or in contact with a cleaning solution or some other detrimental consitituent. In many of these applications, especially those involving sanitary requirements for handling food products, it is also necessary for all product contact surfaces to be of a specified non-toxic material. This usually obviates the use of lead, tellurium, and other elements for even minor alloying additions. Additionally, many sanitary requirements, such as those in the dairy industry, do not allow or discourage the use of copper as an alloying element due to objectionable chemical interaction with various product enzymes. These types of requirements drastically limit the available selection of materials for corrosion-wear-sanitary conditions, eliminating entire classes of bearing-type materials; i.e., copper base brasses and bronzes, babbit materials, liquid self-lubricating composites and the like.
Practically, it is overwhelmingly desirable to employ an austenitic stainless steel alloy for at least one element in any mating couple of the foregoing kind due to the wide acceptance of stainless steel in meeting corrosion and sanitary requirements and economy of manufacture.
Austenitic stainless steel, however, when self-mated in most any form of intermittent or continuous metal-to-metal moving contact exhibit a galling tendency in anything less than full hydrodynamic lubrication conditions. Non-lubricated or boundary conditions abound in material coupled used in the above applications; frequently the only lubricant is the handled media itself which can actually be, in certain instances, hostile to the surface interaction condition due to its abrasive and/or corrosive nature.
Prior approaches to this problem have involved three basic actions:
1. Designs to eliminate severe contact through adequate clearancing, loading factors, and the like;
2. Surface coatings, overlays, or treatments of the base materials to provide acceptable surface mating characteristics;
3. Monolithic alloy structures, not necessarily single phase, formed by the normal fabrication methods and which, by composition, provide acceptable self-mating or couple mating with a second component, usually stainless steels.
Of the obviously desirable latter two approaches a well knwon example may be cited, namely, Thomas and Williams U.S. Pat. No. 2,743,176, which teaches the manufacture of a nickel base casting alloy containing bismuth. The resulting alloy provides most of the requirements listed heretofore but poses some disadvantages of castability due to its wide freezing range and the tendency of bismuth to evaporate. The bismuth addition does freeze, although inhomogeneously, as a complex network of elemental bismuth, together with several nickel-bismuth intermetallic phases, through peritectic reaction in the nickel based alloy system, providing exceptional galling resistance when the alloy is mated with austenitic stainless steels, explainable by surface interaction principles related to adhesive wear beyond the scope of this disclosure.
The Thomas and Williams alloy has long been an industry standard. It is employed as a standard for comparison in Lynch et al U.S. Pat. No. 3,671,207 which discloses a composition similar to the Thomas casting alloy for use in a fusion overlay process, with boron and silicon added for hardening and fluxing required for this type of surface deposition, but simultaneously decreasing its machinability. The Lynch technique of physical overlay is limited when parts become complex in shape and contact surfaces become inaccessible or difficult to coat uniformly.
Other alloys of prior art, formed by any of the above mentioned methods of fabrication, generally rely on the use of compositions and phase structures that allow for acceptable self-mating or mating with an alloy other than an austenitic stainless steel. Examples are the nickel base alloys of Johnson U.S. Pat. No. 2,930,786, and the iron base alloys of U.S. Pat. No. 3,912,503. In most every instance, these materials will gall badly when mated with austenitic stainless steels at the loading stress, surface velocity, and lubrication conditions commonly encountered in the equipment of interest.